This invention relates to a Stator yoke and a method of producing the stator yoke for stepping motors, for example.
Stator yokes and frame yokes of conventional stepping motors are produced by using an electromagnetic soft magnetic iron plate (SUY), a cold rolled low carbon steel sheet (SPC), electrolytic zinc-coated steel sheet (SEC), and so forth. However, although these materials have excellent D.C. magnetic field characteristics, it has been found out that their A.C. magnetic field characteristics are inferior during the practical operation of the motors. In other words, because these steel materials have low electric resistivity, a great deal of eddy currents occur, so that an eddy current loss remarkably increases. This tendency becomes more remarkable as a driving frequency is higher. Accordingly, efficiency drops and this constitutes a bottle neck in satisfying the requirement for the improvement of efficiency of the stepping motors in connection with a battery-driven OA equipment.
To solve such problems, various proposals have been made. For example, a silicon steel sheet or soft ferrite is used for the stator yoke and the frame yoke (Japanese Utility Model Laid-Open Nos. 3-104077/1991, 62-135577/1987, etc.), and a through-hole is formed in a part of the stator yoke (a flow path of the eddy current) (Japanese Patent Laid-open No. 3-283049/1991, etc.).
However, the conventional stepping motors described above involve the following respective problems and cannot sufficiently solve the problem of the drop of efficiency due to the eddy current.
First, in the case of the silicon steel sheet, bending is more difficult than with SUY and SPC used for the stator yoke material, and plating treatment for rust-proofing is difficult. It might be possible to reduce the amount of addition of Si so as to improve machinability but in this case, electric resistivity drops, too, so that the effect of the reduction of the eddy current disappears.
When the soft ferrite is used, a saturation flux density is as low as 1/3 that of SUY and SPC. Therefore, in addition to the problem that a sufficient output torque cannot be obtained, there occur various other problems that bending is difficult, mechanical strength is low and moreover, dimensional precision drops below a practical level.
According to the construction wherein the through-hole is bored, the output torque tends to decrease with the formation of the through-holes. Furthermore, the number of machining steps increases and the production cost becomes higher. The through-holes are formed at portions other than comb tooth-like magnetic pole portions other than the comb tooth-like pole portions. In other words, the effect brought forth by the formation of the through-hole is not great because the loss due to the eddy current is originally not so great at such portions.
Accordingly, the inventors of the present invention previously developed a stepping motor capable of solving these problems, reducing the occurrence of the eddy current to minimum and obtaining high efficiency, and filed a patent for this stepping motor in our Patent Application No. 5-3099698/1993, filed Nov. 17, 1993. In this Japanese application a part, or the whole, of at least the stator yokes constituting the stepping motor is made of a ferrite type Fe-Cr type alloy that has never been used for the stator yoke in the past. (A specific construction will be later described.) The invention disclosed in the Japanese application constitutes an embodiment of the present invention.
When the stator yoke is produced using the new material which has not been used in the past and the stepping motor is assembled and produced using such stator yokes, the effects of restriction of the eddy current, etc., could be observed, but subsequent studies have revealed other problems. Namely, when the stator yoke is produced, a flat sheet material having a predetermined shape is press worked, but this press working invites deterioration of magnetic characteristics and the remarkable effect brought forth by the use of the Fe-Cr type alloy cannot be exhibited sufficiently though performance of the motor can be improved as a whole because the effect of restriction of the occurrence of the eddy current is relatively greater. Therefore, as well as development of the new material for the stator yoke, that is, the ferrite type Fe-Cr type alloy, the development of the production method suitable for producing the stator yoke by using the Fe-Cr type alloy has become necessary at the same time.